Grade

Likely Uses and Characteristics*

Chemical Quality

Dissolved oxygen (%saturation) 10-percentile

Biochemical oxygen demand (mg/l) 90-percentile

Ammonia (mgN/l)

A-Very good

All abstractions

80

2.5

0.25

Very good salmonid fisheries

Cyprinid fisheries

Natural ecosystems

B-Good

All abstractions

70

4

0.6

Salmonid fisheries

Cyprinid fisheries

Ecosystems at or close to natural

C-Fairly good

Potable supply after advanced treatment

60

6

1.3

Other abstractions

Good cyprinid fisheries

Natural ecosystems, or those corresponding to good cyprinid fisheries

D-Fair

Potable supply after advanced treatment

50

8

2.5

Other abstractions

Fair cyprinid fisheries

Impacted ecosystems

E-Poor

Low grade abstraction for industry

20

15

9

Fish absent or sporadically present, vulnerable to pollution**

Impoverished ecosystems**

F-Bad

Very polluted rivers which may cause nuisance

<20

-

-

Severely restricted ecosystems

*Provided other standards are met

**Where the grade is caused by discharges of organic pollution